Brewmaster's Warehouse, William's Brewing, MoreBeer, Austin Homebrew, Midwest Supplies, Northern Brewer, Label Peelers (for hops especially), Hops Direct (for hops especially), Adventures in Homebrewing (a.k.a., Homebrew.org), Brewers Hardware and Bargain Fittings for fittings.....you really can't go wrong with any of these. I've ordered from them all, and had great experiences with each. I've even had a few boo boos with almost all of them, and they all make it right ASAP.
No one shop has stuff for "the cheapest" across the board. You always need to shop around to find the best prices on any given item, but all of these have great selection, quality items, fresh ingredients, and good customer service.
For things like StarSan, always check eBay or do a Google shopping search as well. Anything that is not strictly a homebrew item and has different applications is often a bit cheaper when it's not specifically marketed as a homebrew item, like sanatizer, cleaners, pumps, fittings, pots, extract flavoring, spices, etc., so it pays to look for those type items on sites other than homebrew sites. Hell, McMaster Carr/Grainger is most of the DIYer's favorite homebrew hardware supply store.
Also, don't forget your local homebrew shop!! Most LHBSs are a bit pricier than the online guys, but you don't have to pay for shipping AND if it is just a couple bucks difference, it pays off long term to support a local business and build local relationships in your local homebrew community. In general, items like carboys, corny kegs, and bulk grain are ALWAYS going to be cheaper locally because they cost a ton to ship.
Good luck!